Twenty five years ago it was discovered that a sound wave in the lithium niobate crystal can diffract and rotate the plane of polarization of a polarized light beam with high efficiency up to high frequency. The advances in the high frequency acoustic techniques and laser technology have put many applications into reality from these phenomena and a variety of acousto-optic devices such as acoustic delay lines, acousto-optic tunable filters, modulators and display devices could be built. These devices require unique properties to achieve high performance. For example, a high performance signal-processing device is characterized by a large information processing capability measured by the time bandwidth product of the device. For an acousto-optic Bragg cell, the large time bandwidth product is achieved by extremely wide band operation and long time delay. The properties that make any material important are (a) spectral transmission range, (b) photo-elastic coefficient, (c) acousto-optic figure of merit, (d) acoustic velocity, and (e) acoustic attenuation. The acousto-optic figure of merit for a material is defined as: EQU M.sub.2 =n.sup.6 .multidot.p.sup.2 /d.multidot.v.sup.3 ( 1)
where n is the refractive index, p is the photo-elastic constant, d is the density and v is the acoustic velocity. The figure of merit M.sub.2 is the measure of diffracted light efficiency for a given power. For a good candidate material the refractive index and photoelastic coefficient should be high and density and acoustic velocity and acoustic attenuation should be very low.
Several materials have been proposed and used for acousto-optic devices. Although some of these materials are suitable for certain applications each has limitations. For example, commercially available crystal lithium niobate, tellurium dioxide and lead molybdate are not suitable for light beams in the 0.9 to 10 micrometer wave length region.